Examples of typical known systems for transmitting power between apparatuses arranged close to each other include magnetic-field-coupling power transmission systems in which power is transmitted from the primary coil of a power transmitting apparatus to the secondary coil of a power receiving apparatus using a magnetic field. However, when power is transmitted using magnetic coupling, since electromotive force is strongly influenced by the magnitude of magnetic flux passing through each coil, high accuracy is required in the relative positional relationship between the primary coil and the secondary coil. In addition, since coils are used, it is difficult to reduce the sizes of the apparatuses.
On the other hand, electric-field-coupling wireless power transmission systems are known, as disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-531009 and Japanese Unexamined Patent Application Publication No. 2009-296857. In these systems, power is transmitted from the coupling electrode of a power transmitting apparatus to the coupling electrode of a power receiving apparatus through an electric field. This method allows the accuracy of the relative positional relationship between the coupling electrodes to be relatively low and allows the sizes and thicknesses of the coupling electrodes to be reduced.
FIG. 1 illustrates the basic configuration of the power transmission system disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-531009. This power transmission system includes a power transmitting apparatus and a power receiving apparatus. The power transmitting apparatus includes a high-frequency high-voltage generator circuit 1, a passive electrode 2, and an active electrode 3. The power receiving apparatus includes a high-frequency high-voltage load circuit 5, a passive electrode 7, and an active electrode 6. As a result of the active electrode 3 and the active electrode 6 being arranged to be close to each other with a gap 4 therebetween, these two electrodes are coupled to each other through an electric field.
In the power transmission system disclosed in Japanese Unexamined Patent Application Publication No. 2009-296857, a power transmitting apparatus includes a first resonant circuit that resonates with an AC signal generated by an AC signal generator and a power feeding electrode. A power receiving apparatus includes a power receiving electrode that generates an electric signal, a second resonant circuit that resonates with the electric signal, a rectifier that generates a DC power from the electric signal with which the second resonance circuit is resonating, and a circuit load. The active electrode and passive electrode of the power transmitting apparatus are arranged in the same plane, and the active electrode and passive electrode of the power receiving apparatus are arranged so as to face the corresponding electrodes of the power transmitting apparatus with a predetermined separation therebetween.
In the power transmission system disclosed in Japanese Unexamined Patent Application Publication
(Translation of PCT Application) No. 2009-531009, the active electrodes of the power transmitting apparatus and the power receiving apparatus are made to be close to each other, thereby forming a strong electric field between the electrodes, and the capacitance generated between the passive electrodes of the power transmitting apparatus and the power receiving apparatus is made to be as high as possible. Hence the sizes of the passive electrodes need to be increased. When the passive electrode of the power transmitting unit, the active electrode of the power transmitting unit, the active electrode of the power receiving unit, and the passive electrode of the power receiving unit are arranged in a vertical direction in a narrow vertically-long space, stray capacitance is likely to become excessively high. Also in the power transmission system disclosed in Japanese Unexamined Patent Application Publication No. 2009-296857, since the active electrode and passive electrode are arranged adjacent to each other in a plane, stray capacitance generated between the active or passive electrode and a circuit substrate arranged close to the electrode is likely to become excessively high. Hence, in both of the cases, coupling is not strong and transmission efficiency is low.
Both in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-531009 and in Japanese Unexamined Patent Application Publication No. 2009-296857, capacitance between the passive electrodes of a power transmitting apparatus and a power receiving apparatus cannot be made to be sufficiently large due to these structures, and the smaller this capacitance between the passive electrodes, the larger the fluctuations in the potentials of the two passive electrodes. The fluctuations in the potentials of the passive electrodes of the power transmitting apparatus and the power receiving apparatus may cause leakage of an undesirable electromagnetic field or fluctuations in the ground potentials of the power transmitting apparatus and the power receiving apparatus.